Molecular Mechanisms That Link Oxidative Stress, Inflammation, and Fibrosis in the Liver

Abstract

Activated hepatic stellate cells (HSCs) and myofibroblasts are the main producers of extracellular matrix (ECM) proteins that form the fibrotic tissue that leads to hepatic fibrosis. Reactive oxygen species (ROS) can directly activate HSCs or induce inflammation or programmed cell death, especially pyroptosis, in hepatocytes, which in turn activates HSCs and fibroblasts to produce ECM proteins. Therefore, antioxidants and the nuclear factor E2-related factor-2 signaling pathway play critical roles in modulating the profibrogenic response. The master proinflammatory factors nuclear factor-κB (NF-κB) and the nucleotide-binding oligomerization domain (NOD)-like receptor protein 3 (NLRP3) inflammasome may coordinate to produce and activate profibrogenic molecules such as interleukins 1β and 18, which effectively activate HSCs, to produce large amounts of fibrotic proteins. Furthermore, the NLRP3 inflammasome activates pro-caspase 1, which is upregulated by NF-κB, to produce caspase 1, which induces pyroptosis via gasdermin and the activation of HSCs. ROS play central roles in the activation of the NF-κB and NLRP3 signaling pathways via IκB (an inhibitor of NF-κB) and thioredoxin-interacting protein, respectively, thereby linking the molecular mechanisms of oxidative stress, inflammation and fibrosis. Elucidating these molecular pathways may pave the way for the development of therapeutic tools to interfere with specific targets.

Keywords: NF-κB; NLRP3 inflammasome; NRF2-KEAP1; ROS; extracellular matrix; hepatic stellate cells; inflammation; liver damage.

Figure 1

Reactive oxygen species (ROS) can induce the activation of hepatic stellate cells (HSCs), leading to the deposition of extracellular matrix (ECM) proteins, fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). ROS promote the activation of HSCs from a quiescent state, which is the ECM-producing phenotype; in turn, ECM deposition leads to fibrosis, cirrhosis, and eventually HCC. Antioxidants, nuclear factor-E2-related factor-2 (NRF2) and nitric oxide (NO) seem to play antifibrotic roles by inhibiting ROS-induced HSC activation. Inducible nitric oxide synthase (iNOS) can synthesize large amounts of NO in the liver, utilizing L-arginine as a substrate.

Figure 2

The nuclear factor-E2-related factor-2 (NRF2) pathway exerts antifibrotic effects. Reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂), superoxide anion (O₂^●−), hydroxyl radical (^●OH), alkoxyl radical (RO^●), peroxyl radical (ROO^●), and singlet oxygen (¹O₂), can activate hepatic stellate cells (HSCs) directly or by triggering inflammation or programmed cell death (PCD). In turn, activated HSCs produce large amounts of extracellular matrix (ECM) proteins that are conducive to fibrosis. Cells can regulate ROS by inducing the endogenous NRF2 antioxidant system. Inactive NRF2 is bound to the primary oxidative-stress-sensor and cytoskeleton-binding protein Kelch-like erythroid cap-n-collar (CNC) homolog-associated protein 1 (KEAP1) in the cytoplasm. Activated NRF2 is liberated from KEAP1; in turn, free NRF2 translocates to the nucleus and dimerizes with one of the small Maf (sMaf) proteins and binds to the antioxidant response element (ARE), upregulating antioxidant genes, including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), reduced glutathione (GSH), heme oxygenase-1 (HO-1), NAD(P)H dehydrogenase (quinone 1) (NQO-1), and glutamate-cysteine ligase catalytic subunit (GCLC), which are effective in decreasing ROS; therefore, the NRF2 signaling pathway exerts an indirect antifibrotic effect.

Figure 3

Reactive oxygen species (ROS) trigger fibrosis via several inflammatory pathways. Alcohol, viruses, or other toxic stimuli may injure hepatocytes, which release ROS and damage-associated molecular patterns (DAMPs) that bind to Toll-like receptor 4 (TLR4) in Kupffer cells. Then, the TLR4/MyD88 pathway activates nuclear factor-κB (NF-κB), which triggers the inflammatory response by promoting the transcription of the nucleotide-binding oligomerization domain (NOD)-like receptor protein 3 (NLRP3), pro-caspase-1, pro-IL-18, and pro-IL-1β. NLRP3 induces the maturation of caspase 1, which in turn triggers inflammasome activation mediated by ROS and apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC); inflammasome activation induces the maturation of IL-1β and IL-18, which promote the activation of hepatic stellate cells (HSCs). ROS can activate the inflammasome through the thioredoxin-interacting protein (TXNIP) signaling pathway. NF-κB may promote caspase 11 upregulation via the IFN/STAT pathway. Caspase 1 and caspase 11 can induce pyroptosis via gasdermin by canonical or noncanonical pathways, respectively. Pyroptosis induces the activation of HSCs. ROS may promote NF-κB activation via the dissociation of the inhibitory factor IκB. Importantly, the activation of HSCs by any mechanism results in liver fibrosis.